Several DNA-directed in vitro protein synthesizing systems have been developed to study the regulation of gene expression..sup.1,2 Most laboratories use a modification of the original unfractionated system described by Zubay and coworkers,.sup.3 but a highly defined system has also been described..sup.4,5 In most cases, the synthesized complete protein product is assayed by using gel electrophoresis and/or immunoprecipitation techniques; procedures that are long and rather complicated and often involve numerous reagents. In some instances, enzymatic assay of the product is possible, e.g. .beta.-galactosidase,.sup.3 but this depends on the gene product and the activity of the in vitro synthesizing system.